1. Field of the Invention
The present invention relates to an indicating instrument for a vehicle.
2. Description of Related Art
Conventionally, an indicating instrument for a vehicle that rotates a pointer by decelerating and transmitting to the pointer the rotation of a step motor through a reduction gear mechanism, is known. The pointer indicates a vehicle state value, which is indicated with a zero value as a reference, in accordance with a rotational position. An indicating instrument for a vehicle that rotates a pointer through the application of a drive signal of an alternating current, which alternates in accordance with an electrical angle, to a field winding of a step motor, is described in, for example, Japanese Patent No. 3770095 (corresponding to U.S. Pat. No. 6,624,608 B2).
In the indicating instrument for a vehicle according to Japanese Patent No. 3770095, the pointer that is rotated in a zero-reset direction, which is a direction to return the pointer back to a position of a zero value, is stopped at a stopper position by a stopper mechanism. While the pointer is rotating in the zero-reset direction, an induced voltage is generated in the field winding of the step motor. When the pointer stops, the induced voltage decreases. Accordingly, in zero-reset control that rotates the pointer in the zero-reset direction through drive signal control from a waiting point, which is a predetermined electrical angle, if a detected voltage of the induced voltage is equal to or smaller than a set value, it is determined that the pointer has stopped at the stopper position, and an electrical angle that corresponds to this stopper position is updated. As a result, even if the step motor loses synchronization due to disturbance such as vibration prior to start up of the instrument, so that a rotational position of the pointer is misplaced, the rotation of the pointer is controlled based on the updated electrical angle.
In a reduction gear mechanism that constitutes the indicating instrument for a vehicle of Japanese Patent No. 3770095, a backlash inevitably exists between gears that are in engagement with each other. For that reason, even though the step motor starts to rotate by a start of the zero-reset control from the waiting point, the engagement between these gears may be retarded because of the backlash between the gears in the reduction gear mechanism. In this case, because a speed of the rotation of the step motor is reduced in accordance with formation of an engagement state between the gears, as illustrated in a first diagram of FIG. 15, after the rotation of the motor is delayed relative to a target that corresponds to the drive signal, the speed of the rotation of the motor is increased so as to make up for the delay. When a rotational speed of the step motor (i.e., angular velocity in a second diagram in FIG. 15) exceeds the target speed as a result of such a speed increasing operation, even though the pointer rotates to the stopper position, the detected voltage of the induced voltage is not equal to or smaller than the set value, and the electrical angle that corresponds to the stopper position may be erroneously updated. Therefore, due to a drive signal based on the erroneously updated electrical angle, an unexpected loss of synchronization of the step motor, for example, is caused, so that the rotation of the pointer cannot be accurately controlled. Accordingly, it is hoped that the indicating instrument is improved in the above-described respects.